fixes the grammatical mistake in README.md

[THEFZNKHAN]

• At least two implementers are interested (and none opposed):
  • fix the typos in README.md
  • fix the grammatical mistake in the README.md
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

[THEFZNKHAN]

@domenic
Please review my PR
and if anything needs to change then tell me 😊.

[domenic]

This is not a mistake. Please refrain from submitting such spam PRs in the future.